The utilization of implanted hearing instruments continues to increase with improving technology. Such implantable hearing instruments provide operative and cosmetic utilitarian features relative to conventional ear canal hearing devices. For example, implantable hearing devices offer operative utilitarian features in relation to patients having certain types of conductive or sensorineural hearing loss (e.g., mixed hearing loss comprising a conductive loss component of 45 dB or more with sensorineural hearing loss component of 40 dB or more). These patients are generally known to perform poorly with conventional hearing aids because their conductive and sensorineural hearing loss components are additive and these patients require substantial amounts of gain and output for proper speech recognition.
Conductive hearing loss can happen when there is a problem conducting sound waves anywhere along the route through the outer ear, tympanic membrane (eardrum), or middle ear (ossicles) and is sometimes termed middle ear hearing loss. Sensorineural hearing loss occurs in the inner ear and/or neural pathways. In patients with sensorineural hearing loss, the external and middle ear can function normally (e.g., sound vibrations are transmitted undisturbed through the eardrum and ossicles where fluid waves are created in the cochlea). However, due to damage to the pathway for sound impulses from the hair cells of the inner ear to the auditory nerve and the brain, the inner ear cannot detect the full intensity and quality of the sound. Sometimes conductive hearing loss occurs in combination with sensorineural hearing loss. In other words, there can be damage in the outer or middle ear and in the inner ear or auditory nerve. When this occurs, the hearing loss is sometimes referred to as a mixed hearing loss.
In instances of middle ear or mixed hearing loss, bone conduction devices, such as bone anchored hearing instruments, provide an option for patients in addition to standard hearing instruments or middle and inner ear hearing instruments. Bone anchored hearing instruments utilize a surgically implanted abutment to transmit sound by direct conduction through bone to the inner ear, bypassing the external auditory canal and middle ear. Accordingly, in cases where the middle ear is damaged or deformed, bone anchored hearing instruments provide a viable hearing solution that is typically less invasive than either a middle ear hearing instrument or a cochlear implant.
Some types of bone anchored hearing instruments have a bone screw surgically embedded into the skull with a small abutment exposed though the overlying tissue/skin. An external sound processor connects onto this abutment and transmits vibrations in response to a sound signal to the abutment and hence the bone screw. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, thus providing hearing.
There have been attempts to produce an implantable actuator for generating the necessary vibrations where the implantable actuator can be wirelessly coupled to an external sound processor. However, to date, these attempts have resulted in actuators that provide the vibration of frequency and/or amplitude to stimulate hearing in a manner that has limited utility. For instance, provision of adequate stimulation in these systems can require excitation of a large mass to generate vibration of a magnitude necessary to simulate hearing. This is especially true at low frequencies. Displacement of such a large mass has further complicated efforts due to the high power demands of these devices. That is, as implantable devices typically require a rechargeable battery for energy storage, the power consumption demands of devices utilizing large masses has resulted in devices that do not, inter alia, have an adequate operating duration between charges.